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3-Visitor

## Variable order switched using symbolic substitution not understood

Hi,

When using the symbolic substitution of an expression for a variable in an equation, the order of the variables is getting switched by mathcad 15, which is only version of mathcad I use.  How do I prevent this from happening?

for example

y=x-a

z=y+b

when I copy expression x-a and select y in the 2nd equation & use the symbolic substitution from the pull down menu off the tool bar, the new equation for becomes

z=a-x+b

why is the expression x-a getting multiplied by -1 when it’s symbolically substituted for y in the2nd equation?

2 REPLIES 2
23-Emerald III
(To:CB_9619726)

It is known phenomenon that the output of the symbolic is very hard to control.

There is little you can do about that, other than trying out symbolic options such as collect and simplify.

But the substitution of y with x-a in your example should never reverse it to a-x.

This is what Mathcad 11 gives me:

Note that you can also use live symbolics, when you do:

Success

24-Ruby V
(To:CB_9619726)

@CB_9619726 wrote:

Hi,

When using the symbolic substitution of an expression for a variable in an equation, the order of the variables is getting switched by mathcad 15, which is only version of mathcad I use.  How do I prevent this from happening?

for example

y=x-a

z=y+b

when I copy expression x-a and select y in the 2nd equation & use the symbolic substitution from the pull down menu off the tool bar, the new equation for becomes

z=a-x+b

why is the expression x-a getting multiplied by -1 when it’s symbolically substituted for y in the2nd equation?

I can't duplicate what you describe!

When I try I get z=b-a+x.

That may not be the variable order you'd like to see, but its mathematically correct and nothing got multiplied by -1.

BTW, I would suggest not using the static symbolic via the menu but rather the live symbolic via the Symbolic toolbar.

EDIT: Lucs answer did not show up when I started mine. But at least its clear now that not only his MC11 with Maple returns a correct result but also MC15 with muPad which I used 😉

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